Kenneth J. Leedle

Demonstration of electron acceleration in a laser-driven dielectric microstructure

The enormous size and cost of current state-of-the-art accelerators based on conventional radio-frequency technology has spawned great interest in the development of new acceleration concepts that are more compact and economical. Micro-fabricated dielectric laser accelerators (DLAs) are an attractive approach, because such dielectric microstructures can support accelerating fields one to two orders of magnitude higher than can radio-frequency cavity-based accelerators. DLAs use commercial lasers as a power source, which are smaller and less expensive than the radio-frequency klystrons that power today’s accelerators. In addition, DLAs are fabricated via low-cost, lithographic techniques that can be used for mass production. However, despite several DLA structures having been proposed recently, no successful demonstration of acceleration in these structures has so far been shown. Here we report high-gradient (beyond 250 MeV m−1) acceleration of electrons in a DLA. Relativistic (60-MeV) electrons are energy-modulated over 563 ± 104 optical periods of a fused silica grating structure, powered by a 800-nm-wavelength mode-locked Ti:sapphire laser. The observed results are in agreement with analytical models and electrodynamic simulations. By comparison, conventional modern linear accelerators operate at gradients of 10–30 MeV m−1, and the first linear radio-frequency cavity accelerator was ten radio-frequency periods (one metre) long with a gradient of approximately 1.6 MeV m−1 (ref. 5). Our results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems. This would enable compact table-top accelerators on the MeV–GeV (106–109 eV) scale for security scanners and medical therapy, university-scale X-ray light sources for biological and materials research, and portable medical imaging devices, and would substantially reduce the size and cost of a future collider on the multi-TeV (1012 eV) scale.

Dielectric laser acceleration of sub-100 keV electrons with silicon dual-pillar grating structures

We present the demonstration of high gradient (370 MeV/m) laser acceleration and deflection of sub-relativistic electrons with silicon dual pillar grating structures using both evanescent inverse Smith-Purcell modes and coupled cosh-like modes.

Optical gating and streaking of free electrons with sub-optical cycle precision

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

Transverse and longitudinal characterization of electron beams using interaction with optical near-fields

We demonstrate an experimental technique for both transverse and longitudinal characterization of bunched femtosecond free electron beams. The operation principle is based on monitoring of the current of electrons that obtained an energy gain during the interaction with the synchronized optical near-field wave excited by femtosecond laser pulses. The synchronous accelerating/decelerating fields confined to the surface of a silicon nanostructure are characterized using a highly focused sub-relativistic electron beam. Here the transverse spatial resolution of 450 nm and femtosecond temporal resolution of 480 fs (sub-optical-cycle temporal regime is briefly discussed) achievable by this technique are demonstrated.

Sub-optical-cycle control of free electrons by optical near-fields

In this contribution we report on research of the interaction between optical near-fields of periodic nanostructures and free-electron beams with potential application in future miniaturized laser-based accelerating devices [1, 2], in ultrafast electron microscopy or diffraction experiments [3, 4] or in photon-induced near-field electron microscopy [5]. Here we experimentally demonstrate a technique allowing sub-optical cycle temporal gating and streaking of electrons at sub-relativistic energies (25–30 keV). A focused electron beam interacts with the near-field mode induced by infrared femtosecond laser pulses on the surface of a silicon nanograting. The field pattern above the surface of a periodic structure can be decomposed to its spatial Fourier components, which propagate along the surface with different phase velocities. Synchronization of the phase velocity of a particular spatial harmonic with the velocity of the co-propagating electrons leads to efficient energy transfer between the laser field and electrons [1, 2]. As this interaction is linear in electric field, the temporal structure of the oscillating electromagnetic field of the femtosecond laser pulse is imprinted to the electron beam energy and/or transverse momentum with sub-cycle precision (200 as in this experiment [6]).

Simple, picojoule-sensitive ultraviolet autocorrelator based on two-photon conductivity in sapphire

We present a simple autocorrelator for picojoule 226–278 nm pulses from femtosecond-picosecond laser oscillators based on two-photon conductivity in sapphire. The sub-20 W peak power sensitivity is over 10X better than previous UV autocorrelators.

Towards a photonic crystal mode-locked laser

For a given average power, the energy per pulse of a mode-locked laser increases with increasing cavity length, lowering the repetition rate. Photonic crystal slow light optical waveguides can be used to address the high repetition rates and resulting low pulse energies of conventional semiconductor lasers by substantially increasing the effective optical cavity length while keeping the device compact. Such a device could enable a semiconductor laser to power two-photon microscopy, an advanced non-linear technique for time-resolved deep-tissue imaging. We present a design for realizing a monolithic two-segment quantum dot passively mode-locked photonic crystal laser. The cavity consists of a novel photonic crystal waveguide designed for low dispersion and wide bandwidth by engineering the photonic crystal lattice structure. Group velocity dispersion of 2x104 ps2/km, more than an order of magnitude lower than similar dispersion engineered photonic crystal waveguides, is achieved over 2% bandwidth, more than sufficient for mode-locking. Gain is achieved by optically pumping epitaxially grown InAs/GaAs quantum dots in part of the photonic crystal waveguide, and the saturable absorber section is reversed biased to enable pulse shaping. A cladding scheme is used to apply reverse bias to the saturable absorber and shorten its recovery time. Devices are fabricated using a combination of electron beam lithography, anisotropic etching, and selective under-etching processes, similar to standard photonic crystal waveguides. The low-dispersion, wide bandwidth waveguide, combined with the fast dynamics of InAs quantum dots could enable a compact, low repetition rate mode-locked laser to be realized.